Microelectromechanical system (MEMS) microphones are deployed in various devices including consumer electronics and vehicles among other applications and host devices. Such microphones include an assembly of one or more transducers and one or more integrated circuits typically disposed in a housing formed by a lid or cover fastened to a substrate having an external device interface. The transducer is embodied as a MEMS die that detects pressure changes within the housing via an acoustic port through the lid or substrate and the interface is typically configured for surface mounting on a printed circuit board using reflow soldering or other technology.
Some MEMS microphones output a digital signal having an idle tone frequency based on a frequency of the digital circuit clock. For example, PDM microphones output a 1-bit PDM signal including an idle tone frequency that is one-half of the clock frequency. In stereo and other applications where two or more microphones are employed, mixing of the idle tone frequencies or other effects may cause unwanted noise at the output of one or both microphones. The idle tone frequencies may be slightly different due to manufacturing variations or other causes.
FIG. 1 depicts a first plot 100 showing different idle tone frequencies generated by left and right microphones in a stereo system in the frequency spectrum. In particular, the first plot 100 depicts a first idle tone frequency 104 (labeled “f-idle-1”) and a second idle tone frequency 106 (labeled “f-idle-2”) generated by left and right microphone signals, respectively, in a stereo system. FIG. 2 depicts a second plot 200 illustrating an intermodulation (IM) component 208 (labeled “Δf”) resulting from mixing the different idle tone frequencies f-idle-1 104 and f-idle-2 106 in FIG. 1, wherein the IM component 208 lies in the operational frequency range 210 where it may cause noise or interference.
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure.